Device for irradiating objects with neutrons, especially for depicting them by meansof neutron-radiation



June 23, 1942. H. I. KALLMANN El'AL 2,237,620 DEVICE FOR IRRADIATINGOBJECTS WITH NEUTRONS, ESPECIALLY FOR DEPICTING THEM BY MEANS OFNEUTRON-RADIATION Filed June 8. 1940 Patented June 23, 1942 unirso s'rDEVICE FOR IBRADIATING OBJECTS WITH NEUTRONS, ESPECIALLY FOR DEPICTINGTHEM BY MEANS OF NEUTRON -RADIA- 'rroN Hartmut Israel Kallmann,Berlin-Charlottenburg,

and Ernst Kuhn, Berlin, Germany Application June 8, 1940, Serial No.339,561 In Germany June 9, 1939 8 Claims. (c1. 250-51) of anion-collector upon which the ions impinge and where they produceneutrons. For employing the source of neutrons it should be mostconvenient to connect said collector to ground, if possible. With suchan arrangement all other parts of the tube would be on a very highpotential relative to earth; especially the source of ions would showthe full acceleration potential relative to the earth. In order toreduce the potentials occurring relative to earth it has already beensuggested to ground the electrode having the middle potential and lyingbetween those two electrodes that possess the full potential differencebetween each other. With this known arrangement the source of ions, aswell as the source of neutrons, each has relative to earth, only onehalf of the whole accelerating potential. If this potential is veryhigh, for instance 1 million volts or even more, it is with such anarrangement necessary to choose a very large distance between the sourceof neutrons and the objects to be irradiated, whereby the radiationintensity actually applied to the objects will be strongly reduced.

It is an object of the present invention to provide aneutron-irradiating device which is free from this drawback. This andother inventive objects are attained by arranging the source ofneutrons, which source has a high potential relative to earth, insidethe protection space of a discharge shield, and by housing the objectsto be irradiated also inside the protection space of another dischargeshield located opposite the firstmentioned one and being preferablygrounded. By the expression discharge shield is here meant a conductivecasing of the greatest possible radius of curvature and free from edgesor projections which would facilitate discharge. While with the usualarrangement, if a potential of, say, 800,000 volts is employed adistance of nearly 3 meters is necessary to obviate flashingover, thenew arrangement of this invention requires under the same conditions asregards potential and earth a distance of only 40 centimeters.Furthermore, owing to the circumstance that the objects to be irradiatedare likewise enclosed in the protection space of a discharge shield,there is no danger whatever for the attendants and no risk of theobjects being damaged even if flashing-over should occur.

'The means for slowing down the quick neutrons being preferably housedinside the discharge shield containing the source of quick neutrons arethus also protected from any damage by flashing over. Therefore thedistance between the two discharge shields can be made so small that ingeneral no flashing over occurs. It is by no means necessary to choosethis distance considerably larger for reasons of safety. In general itis likewise suitable to provide the means for obtaining a narrower beamof neutrons also inside the discharge shield containing the source ofneutrons.

It is frequently desirable to protect the objects to be irradiated fromthe action of X-rays which arise unavoidably if neutrons are produced.It is therefore recommended to provide one or both of the two dischargeshields especially on their outer surface, with layers of substancescapable of absorbing X-rays. If the objects are to be irradiated withslow neutrons or are to be depicted by means of slow neutrons, it isadvantageous also to arrange the means for slowing down the speed of thequick neutrons also inside the protection space of the discharge shieldcontaining the source of the quick neutrons. In

order to produce a sharply limited beam of slow neutrons it is ingeneral suitable to arrange these means in such a manner that the slowneutrons leave the discharge shield by preference from a narrowlyrestricted range thereof, and it is, further, more advantageous toprovide additional means for absorbing the slow neutrons in theprotection space, in the wall thereof, or/and upon its outer surfaces.In order to obviate a loss of intensity the thickness and the materialof the wall of the discharge shield within the range where the neutronsleave said body may be so chosen that the intensity of the neutrons isas little as possible.

Furthermore, in order to make the distance between the source ofneutrons and the objects to be irradiated as short as possible, it is ingeneral suitable to place the source not in the middle of the-dischargeshield which is rather large in the case of the potential being high,but as near as possible to that part of the wall of the discharge shieldwhere the neutrons leave this body.

The invention is illustrated diagrammatically devices.

and by way of example in the accompanying drawing in which is shown alongitudinal section through a device according to this invention.

The discharge tube consists of three parts. I denotes the source ofions. The positive ions leave the part I at the end 2 and areaccelerated by the field existing between this end and the oppositelylocated end 3. Then the ions pass through the second part, leave it at 4and are again accelerated by-the field existing between the ends 4 and5. The ions leave the third part with a very high speed at 6,-are againaccelerated by the field existing between 6 and l and impinge now uponthe intercepting member 8. This lattor member is arranged within thedischarge shield 9 near the part where the neutrons l tend to leave thisbody. In order to slow down the speed of the neutrons a substance H, forexample water, surrounds the intercepting member. Through a channel [2free from the substance II the slow neutrons pass to the exterior of thedevice. They penetrate the wall of the oppositely located shieldingelectrode l3 and partly also the object! arranged behind said shieldingelectrode. The neutrons having traversed the object produce upon theplate I5 which is sensitive to neutrons, or-upon asuitable fluorescentscreen, an image of the respective object.

The escaping neutrons H] can also be used solelyfor irradiating theobject, in which case the neutron-sensitive device I5 may be removed.The neutron-sensitive system'may be formed for instance by a lithiumfoil covered with a fluorescent substance, or by lithium mixed with sucha substance. The neutrons react with the lithium nuclei liberating heavycharged particle which excite the fluorescent substance. Or a lithiumfoil is arranged in the proximity of a photographic plate, in which casethe emitted charged particles blacken said plate. 1 f

In order to obviate the emission of X-rays there is provided in thedevice illustrated'by the drawing a layer I6 capable of absorbingX-rays, on the outside of the discharge shield 9. This body contains,furthermore, a lining I'I capable of absorbing the slow neutrons andconsisting, for instance, of boron. .The thickness of this layer ispreferably very slight in the proximity of the part where the neutronsleave the body 9, or there is even provided a hole in said liningadjacent the zone of emission. If the layer absorbing X-rays allows theneutrons to pass in a satisfying' degree, it is not necessary to providealso this layer with a separate aperture at the exit place. The distancebetween this place and the electrode l3 amounts to about 40 cm. at apotential of 800,000 volts if the electrode surface is carefullysmoothed.

When designing the source of neutrons as suggested according to thisinveniton'it is not necessary to ground the device in the middle ofpotential difierence, as is done with the known shown three in theexample, have higher potential towards the earth in this arrangementthan ifthey were grounded in the middle of the potential difference, butthis is not attended with any difficulties whatever, as the accelerationstages require only such sources of potential which can be insulated ina comparatively'simple manner. Thus, with a total difference ofpotential of for instance 1.2 million volts, the potential differencesV1, V2 and V3 will be chosen equal to 400,000'volts. If the totalpotential difference should amount to 2 million volts, it would besuitable to. choose also equal to 400,000 volts and to overcometheremaining potential difference of 1.6 million with a correspondingnumber of stages, for instance 4. This arrangement presents,also, thefurther advantage that there exists near the end of the ion-producingtube the possibility of examining the centering of the beam of ionsrelatively to the entrance aperture of the electrode 3 and to the exitaperture 2 through the window l8 shown diagrammatically in the drawing,and of improving the centering if necessary, by shifting the electrodeswith respect to one another. It may be suitable to join the high vacuumpump (not shown) to the apparatus at the place l9, as shown in thedrawing.

We claim:

1. A device for irradiating an object with neutrons, comprising a sourceof neutrons, means for exciting the neutron source, means formaintaining the neutron source at a high potential relative to earth, adischarge shield substantially enclosing the neutron source, and asecond discharge shield adapted to house an object to be irradiated,said second discharge shield being relative to earth, a discharge shieldsubstantially On the contrary, it is particularly suit I able to thepurpose in view to effect the' subdi-" vision in such a manner that thepositive elec--' trode 3 of the appertaining accelerating stage isgrounded.- The device for the production of positive ions than showsonlya comparatively there exist no particular difl'iculties ininsulating the auxiliary source of potential necessary for the.ion-source towards the earth. I v

The accelereating stages,- of which there are enclosing the neutronsource, means within said discharge shield and adjacent said neutronsource for forming a sharply limited emergent beam of neutrons, and asecond discharge shield adapted to house an object to be irradiated,said second discharge shield being arranged opposite to the first nameddischarge shield and in the path of such sharply limited emergent beamof neutrons.

3. A device as defined in claim 1, wherein at least one of the dischargeshields is provided at least partially with a layer of X-rays-absorbingsubstance.

4. A device for irradiating an object with neutrons,'comprising a sourceof quick neutrons, means for exciting the neutron source, means formaintaining the neutron source at a high potential relative to earth, adischarge shield substantially enclosing the neutron source,

means within said discharge shield for slowing down quick neutrons, saidmeans including a body of material capable of slowing down quickneutrons at least partially surrounding said neu- 5. A device as definedin claim 4 in which the means for slowing down quick neutrons isarranged in such a manner that neutrons pass to the exterior onlythrough a limited area of the surface of the first named'dischargeshield.

6. A device as defined in claim 4 in which the discharge shieldcontaining the means for slowing down the quick neutrons is providedwith substances adapted to absorb slow neutrons.

7. A device as defined in claim 2, wherein said beam-forming meanscomprises a layer of slow neutron-absorbing material provided on thefirst named discharge shield and formed to define a neutron passagewayleading from said neutron source to a zone of emergence of saiddischarge shield, said neutron source being disposed in proximity tosaid zone of emergence.

8. A device as defined in claim 2, wherein that portion of the firstnamed discharge shield which is within the range of said beam ofneutrons is composed of such material and has such thickness as toeffect only a minimum of reduction in the intensity of the neutronsemergent therethrough.

HARTM'UT ISRAEL KALLMANN. ERNST KUHN.

